The Judging of Fieldglasses, by Roland Leinhos
Zur Beurteilung von Feldstechern.
Zeiss Werkzeitschrift, 7 (1959), p.78-83
Translated by Ilse Roberts and Peter Abrahams
Text of this translation copyright 1996
(Another translation of this article was published by Zeiss in 1960.)
When discussing field glasses, the optical data, such as magnification
and less frequently the FOV, are the main subjects. With these data one
distinguishes the different field glass models and associates them with
their purpose. But if the layman looks at the pamphlets of different
manufacturers, he is usually surprised to find the same optical
specifications everywhere, at least in regards to magnification & objective
diameter. The only differences he might find are for the F.O.V. (field of
view). But this is not surprising in the least since the specifications
serve as differentiation for various models of a manufacturer and don't say
anything about the other characteristics of the fieldglasses. The field
glass amateur will then usually ask if the difference in the F.O.V. is the
only one, or if one can see further differences in field glasses between
manufacturers. Since this is asked again and again,and there are indeed a
number of characteristics to differentiate, we shall go into more detail.
But we shall stay away from the characteristic distinctions of the Zeiss
fieldglass, as for example the smaller construction and therefore more
comfortable use, or the models with greater eye relief, for wearers of
spectacles.
At first we shall talk about the concept of size of F.O.V. This is one
of the specifications peculiar to an individual field glass, since the
F.O.V. does not depend on magnification and objective diameter, but on the
field stop installed in the focal plane. This is valid with the exception
that for reasons of optical design, the diameter of the f.o.v. becomes
smaller, with higher magnification. But also for fieldglasses with the same
magnification, and the same objective diameter, different F.O.V.s are found.
For example, there are 10 x 50 field glasses, which in a standard
construction have a F.O.V. of 130m at 1000 m distance, corresponding to an
angle of 7.4 degrees on the objective side. But one also finds field glasses
with about 100m (5.8 degrees) or even those that have a F.O.V. of only 88 m
and 112 at 1000m (5 degrees). For certain glasses the F.O.V. is thus 1 1/2
times larger than that of other makes. There is not as much difference with
8 x 30 binoculars, where one can find models with F.O.V. between 150 m (8.5
degrees) and 112 m (6.4 degrees). These are then values which the
manufacturer prints in his pamphlets; and for a quality product it is to be
expected that these values correspond to the experimentally established
ones.
One can easily discern differences in the F.O.V. by mounting the field
glasses on a tripod and pointing them so that at the left rim of the F.O.V.,
a detail of an object with a grid (tile roof, windowpanes, or something like
it) is visible and then counting the details (tiles or panes) towards the
right rim. Of course one can also do such a measurement of F.O.V. in a
larger room ( 7 by 10 m) with a measuring stick. Also, a measuring stick or
similar markings can be attached directly onto a test slide with Foucault
line figures, as in fig. 1.
But the information about the F.O.V. is not enough; as one can easily
see, there are also differences in regards to the sharpness of the image in
the center, in the mid zones and at the rim of the F.O.V. For example,
there are field glasses which, when the center is in focus, have a
distinctly noticeable sharpness decline towards the edge of the field, and
also those which produce a focused image up to the edge. It is obvious,
that the fieldglass with the better edge sharpness does not only produce an
aesthetically more satisfying image, but also facilitates the early
recognition of details at the edge of the F.O.V. when the field glass is
traversed across a view. The edge sharpness does indeed have practical
importance.
(Fig 1. Test plate with Foucault line system. The numbers give a quantity
to the focus, an arbitrary unit of length, that depends on observation
distance. In the depicted size the test plate is appropriate for an
observation distance of about lOm.)
Visual tests of the image quality are not so simple and require some
experience.Such tests are much easier when a testplate with the Foucault
line system can be used (fig. 1). With it, the resolving power can be found
for the center, as well as at the edge or the zone in between. Astigmatism
can be recognized when the vertical and horizontal lines appear sharp at
different dioptre settings of the field glass. Such a procedure is not
totally objective, since the aberrations of the eye are not eliminated and
furthermore the eye movements and adjustments have to be considered. A
secure position of the fieldglass and careful observation are matters of
course.
If the quality test is to be made more exact, a star test instrument is
required as shown in fig 3. A collimator with a pinhole in its focal length
produces an artificial star. The field glass is set in front of the
collimator so it can be rotated around the objective opening. Behind the
fieldglass, instead of the eye is a camera with long focal length (500 mm)
lens. Swiveling and shifting adjustments allow the placement of the image
of the star in the middle of the field glass F.O.V., as well as at the rim
or in the mid zones, and from there to the photo plate.
(Fig. 2. Star tests of various field glasses. A, nearly perfect. B,
Spherical aberration. C, Lens is screwed in too tight. D through F,
centering mistakes of various sizes. G & H, astigmatism. I, mistake from
roof ridge.)
Fig 2 shows some of those star images, as photographed through various
field glasses. In a, the image of a star through a perfect field glass is
reproduced nearly without fault as a round circle. The next image, b, shows
a wide ring around the image proper, which may be caused by incomplete
correction of spherical aberration, or by manufacturing mistakes like
incorrect air spacing or wrong radius of curvature of a lens. The
triangular star image in c is caused by physical distortion of an optical
part (objective or prism). D through f show centering mistakes of various
sizes, which means, the center of curvature of the separate lenses do not
lie on a common line, the optical axis. The images in g and h show
astigmatism, as it appears when the optical planes are not exactly of the
same form, for example the evenness of the reflection planes of the reversal
prisms. I shows the faults that appear through an imperfectly worked roof
prism.
It is obvious that field glasses, which have mistakes in the axis do not
have the resolving power of a good field glass. If one wants to test the
system for resolution with the naked eye after the Foucault test, an
auxiliary telescope should be used.
(Fig. 3. Schematic of an instrument for testing image quality. 1,
collimator. 2, collimator objective. 3, artificial star. 4, light source. 5,
object of examination. 6, entrance pupil of object. 7, exit pupil of object.
8, camera tube. 9, camera objective. 10, film plane.)
In fig 4 are star test photos of an artificial star, at a distance from
the center of the image which is 2/3 of the F.O.V. radius, as shown by
different field glasses. In this figure,too, it can be seen how different
the image sharpness can be. Even the best field glass cannot image a star
at the rim of the F.O.V., or in the mid zone, as a dot, as it does in the
center. The optical designer can minimize this remaining error by a careful
choice of the lenses, radii of curvature, thickness, air spacing, and types
of glass. Fig. 4 gives a few examples and shows qualitative differences.
The two figures to the far right show that because of construction mistakes
the figures can become unsymmetrical, while the size generally is dependent
upon the geometric optical errors. Of course, it is very important for
these figures that the fieldglass in the center has been focused on
infinity.
(Fig 4. Star test photos outside of the center of F.O.V. of different field
glasses. Above left: especially well corrected and perfectly constructed
system.; below left and center: fairly significant remaining mistakes;
right, above and below, distorted figures caused by construction mistakes.)
One can lessen the expansion of the figures, when the fieldglass is
defocused. This adjustment, to the smallest possible number, generally
between minus 0.5 and minus 4 dioptres, gives the F.O.V. a curvature in this
zone; and is also an indicator of the quality of the image.
Now to another matter, concerning the throughput of the field glass, and
consequently the brightness of the glass. We know today that the brightness
of the field glass image is not the only factor in the performance of the
glass at dusk and night, as thought previously. Nevertheless it is
understood that a field glass with reflection diminishing T coating, is
better than one without this coating, since it delivers a brighter image.
According to a law of nature, at each meeting of air and an optical
medium, a certain amount of light is reflected (fig 5); 4% of the incoming
light for a glass with refractive index of 1.5, 7% for a glass with
refractive index of 1.7. By steaming on a thin coating (1/4 wavelength of
light), these losses can be diminished to 1%. The refractive index of the
coating must be between that of the glass and air. A modern field glass has
up to 16 such glass-air surfaces, and without T coating only about half of
the incoming light could exit, while the actual loss for modern coated field
glasses is less than 20%.
If the light loss in itself is undesirable, it is more important that
the light stopped at the surfaces reflects around in the housing and finally
appears in the image as a grey veil; so that a field glass which does not
have an antireflection coating cannot deliver a high contrast, brilliant
image, like one that is properly coated.
(Fig 5. Path of a ray in an uncoated, and a coated lens.)
(Fig 6. Path of a ray in a prism with a small index of refraction.)
Not every coating can be called a compensating coat. A blue coating or
other attractive coating, does not necessarily have the above mentioned
effects. There are also field glasses where only a few of these surfaces
are coated. This is not enough to significantly improve the light
throughput and contrast; for that, it is necessary to coat all surfaces, and
an exception should be made at most for the exterior surfaces of of the
objective and ocular. To measure the effect of the antireflection coating,
exact measurements of the throughput and the decrease in contrast are
necessary, a complicated task.
A further cause of light loss and additional scattering of light can be
the prisms, which serve to reverse the image and shorten the instrument. If
these prisms are made of a glass of an insufficient index of refraction, as
is done quite frequently nowadays, the critical angle for total reflection
of the edge rays is surpassed at the reflecting surfaces of the prisms. The
edge rays from the rim of the F.O.V. strike the reflecting surface of the
prism at a larger angle than the central rays. At these surfaces, light
exits the prism, as the arrows in fig 6 show. At an index of refraction of
1.52, the largest angle of approach for ray which will be totally reflected
by the prism is about 6 degrees. Here, too, brightness and reduction of
contrast are the main problems. If one could perhaps tolerate the loss of
light, so that the effective objective diameter and therefore the
performance at dusk do not correspond to the indicated objective diameter;
the worsening of the contrast is perhaps much more unpleasant.
(Fig. 7. Exit pupil for field glass with prism of glass with too small a
refractive index.)
Even the amateur can easily find out if the critical angle of total
reflection was surpassed at the prisms, by observing the exit pupils. The
exit pupils usually are visible as light round little disks, if one holds
the field glass at about 30 cm distance from the eye against a light
background and looks towards the oculars. Since the ray bundle is being
reflected four times in the prisms, when lower quality glass is used, at
four places the periphery of the pupils are cut. The exit pupil is then not
seen as a round disk, but as a square which is inscribed in the circle. The
cut off segments appear a dim, pale blue, as is indicated in Fig 7.
At this point a word about fastening of the prisms may be allowed. A
secure fastening is especially important for binocular instruments. Even a
tiny shifting of a prism causes misalignment of the light rays, so that
double images appear to the observer. This can happen when bumped and
shaken, which is to be considered normal for field glass usage. For
example, the tipping over of a glass on a table causes forces on the field
glass which correspond to 40 to 80 times the acceleration due to the earth's
gravity (9.81m/s). The allowable tolerance for parallelism of the exit axes
is about 20 arc minutes. Since a manufacturer cannot fit the the prisms
into the housing as exactly as is required, the possibility for adjustment
at the fieldglass is required. For quality field glasses, adjustment with
eccentric rings at the objective mountings is the accepted practice. These
two eccentric rings allow the shifting of the objective within certain
limits, perpendicular to the optical axis in any direction; and consequently
permit the adjustment of the axes of the two field glass halves.
Another means of performing this adjustment, using the prisms, is to
adjust the prisms after the assembly of the whole field glass, mostly from
the outside, by screws through the housing to the edges of the prisms. But
one never has the guarantee that the prism is really fixed in its position,
or is not under such pressure that the prism is warped, thus worsening the
image quality.
A secure prism mounting is the fundamental prerequisite for maintenance
of the binocular alignment over a long period of time and with rough
handling. In early Zeiss field glasses, this fixation of prism position was
achieved by making the housing a little larger than the prisms, which had
two flat grooves on the sides, into which some material from the housing was
pushed with a center punch. In later times, Zeiss switched to retaining
rims, which are placed around the prism and then screwed in. Thus the
prisms are either fastened into the housings individually, or both prisms
are mounted onto a plate, the prism seat, and then put into the housing. In
addition, the prisms are held in position by a spring which presses on the
upper edge, so that they cannot pop out of their seated position when
bumped.
The simplest method of prism fastening is the use of a more or less
suitable cement. Here the main difficulty is the stability of the cement.
It can change because of temperature influences or in a leaking
fieldglasses, weather can loosen the prism.
That brings us to a further point, weather tightness. To protect a
field glass against weather influences, especially the entrance of humidity,
all openings, slits, and pores have to be carefully closed. Humidity can
condense on the optical surfaces, and the fieldglass becomes at least
temporarily unusable. Formerly, wax was used, today, rubber seals are used
for Zeiss field glasses. Special difficulties with center focus models made
the sealing of these field glasses impossible in earlier times. These
glasses now show the effects of their sliding oculars. But this problem was
solved by the introduction of the inverted sleeve for the Zeiss center focus
field glass, fig 8. Here a tube like piece of rubber, folded like a cuff,
was fastened to the ocular seat on the housing and also to the movable
ocular.
(Fig. 8. Cuff seal for Zeiss field glass with center focus. All seals are
drawn in heavy black line.)
In the absence of such a solid seal for the field glass, other methods
of avoiding humidity and condensation are used. These are usually
containers of a drying substance which loses its effectiveness after a short
time, and in the best of cases only protects the field glass for transport
across the ocean.
In conclusion it may be noted that even in the outer appearance of field
glasses from different manufacturers are noticeable differences. These
could be the coverings of the housings; or the engravings at the center
screw, the ocular, or at the trade mark. But even the amateur will be able
to detect the conscientiousness which was used at the factory by examining
the smooth and silky glistening lacquer. Summing up, here are the
characteristics one should keep in mind when comparing field glasses of the
same type.
--the size of the F.O.V.
--the image quality in the middle and at the rim of the F.O.V., as well as
the curvature of the F.O.V.
--the throughput and therefore the brightness of the image, as well as the
contrast, in regards to the antireflection coating.
--the additional loss of light from inferior glass in the prisms (squared
instead of round exit pupils).
--the provision for adjusting collimation, with double eccentric rings at
the objectives. or by positioning the prisms.
--the prism fastening itself, not visible from the outside.
--the sealing of the field glass housing against atmospheric influences; a
well sealed field glass does not need containers of drying substances.
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